This field is really a *m (modulo its bottom bit). Change it from
uintptr to muintptr to document this fact.

Change-Id: I2d181a955ef1d2c1a268edf20091b440d85726c9
Reviewed-on: https://go-review.googlesource.com/46034
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>